Graphene has recently attracted tremendous interests in photonic applications due to its promising optical properties, especially its ability in absorbing light over a broad wavelength range. Graphene, a two dimensional allotrope of carbon atoms on a honeycomb lattice with unique band structure, has recently attracted tremendous interests in photonic applications such as transparent electrodes, optical modulator, polarizer, surface plasmonics and photodetector. One of the better advantages of graphene is the ability to absorb about 2% of incident light over a broad wavelength range despite its thin one-atom layer thickness.
Several works have been previously reported on the realization of broadband graphene-based photodetectors in a field effect transistor (FET) structure. However, the highest responsivity, which can be defined as photo-generated current over per incident optical power, was determined to be lower than 10 mA/W. Although strategies such as using surface plasmons or microcavities have previously been adopted to enhance the performance of such graphene photodetectors, the responsivity obtained was still as low as several tens of milliamperes per watt. Further, such fabrication processes are complex.
In conventional graphene FET-based photodetectors, low responsivity is mainly attributed to the low optical absorption in monolayer graphene, of about 2.3%, and the short recombination lifetime, which is in the scale of picoseconds, of the photo-generated carriers, leading to a low internal quantum efficiency of about 6-16%. In addition, although pure monolayer graphene photodetectors have been predicted theoretically as to be having an ultra-wide band operation, all of the reported results based on presently provided pure monolayer graphene photodetectors did not demonstrate a broadband wavelength coverage range, e.g. from the visible to the mid-infrared.